The present invention is directed to an asymmetrical differential amplifier as a level converter having a first and a second transistor, a constant current source and first and second resistors. A first terminal of the first transistor and a first terminal of the second transistor are connected in common to a first circuit terminal via the constant current source and a second terminal of the first transistor is connected to a second circuit terminal via the first resistor and a second terminal of the second transistor is connected to the second circuit terminal via the second resistor. A first input is connected to a control terminal of the first transistor, a second input (V.sub.ref) is connected to a control terminal of the second transistor and a first output (A) is connected to a second terminal of the first transistor and a second output (A) is connected to the second terminal of the second transistor.
A differential amplifier is a symmetrical DC amplifier having two inputs and two outputs. It is usually constructed with two transistors, whereby one terminal of each of the two transistors is connected to a constant current source. The constant current source provides that the sum of the currents through both transistors remains constant. When a voltage differing in amplitude is applied to the two inputs of the differential amplifier, then the current division in the differential amplifier varies. The current through one transistor increases whereas the current through the other transistor decreases. The sum of the two current magnitudes, however, is equal to the current magnitude that is supplied by the constant current source. An input voltage difference produces an output voltage change in a differential amplifier. A temperature-conditioned change of the voltages at the transistors acts like a common mode modulation and remains ineffective. A differential amplifier is therefore preferably also used as a DC amplifier.
Basic circuits for differential amplifiers have been known from the prior art literature for a long time. The book "Halbleiter-Schaltungstechnik" by Tietze-Schenk, Fifth revised edition, page 59, discloses the basic circuitry of a differential amplifier and the following pages 59-64 show the structure and the functioning of a differential amplifier composed of npn bipolar transistors. In contrast thereto, page 89 and FIG. 5.14 in the same reference show a basic circuit of a field effect differential amplifier, whereby this is constructed with two n-channel barrier [depletion] layer field effect transistors.
Significantly better results with respect to the input current, the bandwidth and the noise can be achieved by using field effect transistors in comparison to the circuit in FIG. 4.32 of the reference. Further particulars regarding field effect differential amplifiers may be found on pages 90-92 in the reference.
The realization of a differential amplifier either with bipolar transistors or with field effect transistors determines the advantages and disadvantages that are critical for the respective transistor form. The realization of a differential amplifier with bipolar transistors or in ECL technology has the disadvantage that the transistors must be insulated from one another. For a differential amplifier in this technology, very low logical output levels as a result of low currents yield a relatively high processing speed. The logical output levels usually lie in the range from 0 through 200 millivolts. Relatively high logical output levels result for a differential amplifier with field effect transistors, for example, using CMOS technique; however, such a circuit is slower in comparison to a differential amplifier that uses bipolar transistors. A relatively high packing density of the individual transistors is possible with the use of CMOS circuit technology. Logical output levels here lie between 0 and 5 volts.